Among fuel cells, direct oxidation fuel cells in which a liquid fuel such as methanol or dimethyl ether is directly supplied to the anode without being reformed to hydrogen gas have been receiving attention. A direct oxidation fuel cell includes a pair of separators and a membrane electrode assembly (MEA) interposed therebetween.
The MEA includes an anode, a cathode, and an electrolyte membrane interposed therebetween. The anode and the cathode each include a catalyst layer and a diffusion layer. To the anode, a fuel and water are supplied, while to the cathode, an oxidant gas (e.g., oxygen gas or air) is supplied. At the anode, the fuel reacts with water to produce carbon dioxide, protons and electrons. The protons pass through the electrolyte membrane to reach the cathode, while the electrons travel through an external circuit to reach the cathode. At the cathode, oxygen reacts with protons and electrons to produce water.
If the produced water accumulates in the cathode in a large amount, the diffusibility of oxidant gas therein is reduced, and the power generation performance of the fuel cell is deteriorated. In order to address this problem, Patent Literature 1 suggests forming a porous composite layer having water repellency, on a surface of the conductive porous substrate constituting the cathode diffusion layer. The constituent material of the porous composite layer disclosed in Patent Literature 1 is a fluorocarbon polymer, such as a copolymer having a tetrafluoroethylene unit and a propylene unit, or a copolymer having a vinylidene fluoride unit and a hexafluoropropylene unit. It is considered that the diffusibility of oxidant gas in the cathode is improved by forming such a porous composite layer.
On the other hand, in view of achieving favorable power generation performance of a fuel cell, it is also important to ensure the diffusibility of fuel in the anode. If the fuel diffusibility is insufficient, the fuel tends to be distributed unevenly in the anode. In this case, in a region where a large amount of fuel is locally distributed, methanol crossover (MCO), a phenomenon in which the methanol used as fuel passes in an unreacted state through the electrolyte membrane to reach the cathode, is likely to occur. The occurrence of MCO leads to an occurrence of fuel loss and reduction in potential at the cathode, resulting in deterioration in power generation performance. On the other hand, in a region where the amount of fuel is small, the power generation reaction does not proceed sufficiently, and the power generation performance is deteriorated.
In order to address this problem, Patent Literature 1 suggests forming a porous composite layer having water repellency on a surface of the conductive porous substrate constituting the anode diffusion layer, as formed in the cathode. It is considered that the uniform diffusibility of fuel in the anode is improved by forming such a porous composite layer.
The catalyst layer includes catalyst metal fine particles or a conductive carbon material (carrier) supporting catalyst metal fine particles, and a polymer electrolyte, and is comparatively highly hydrophilic. On the other hand, according to Patent Literature 1, the porous composite layer as mentioned above, because of the inclusion of a water-repellent material such as a fluorocarbon polymer, tends to have a low critical surface tension. As a result, the interface bonding strength between the catalyst layer and the porous composite layer is likely to be insufficient. In order to ensure a sufficient interface bonding strength between the catalyst layer and the porous composite layer, various studies have been made.
For example, Patent Literature 2 suggests forming a first porous composite layer and a second porous composite layer which is less water repellent than the first porous composite layer, between the catalyst layer and the conductive porous substrate. In Patent Literature 2, the second porous composite layer with poor water repellency is disposed in contact with the catalyst layer.
Patent Literature 3 suggests applying hydrophilic treatment to a surface of the porous composite layer in contact with the catalyst layer, to form a hydrophilic portion, and bonding the hydrophilic portion to the catalyst layer.